1. Field of the Invention
The present invention relates to a planar light source device of an edge light system, and more particularly to a planar light source device which uses a point light source such as a light emitting diode (LED) designed to realize miniaturization and a reduction in power consumption. The planar light source device of the present invention is suitably applied to a backlight of a relatively compact liquid crystal display device used as a display panel of a portable electronic device such as a cellular phone, or an indicator of various devices.
2. Background of the Related Art
Recently, the liquid crystal display device has been in wide use as a monitor for a portable notebook personal computer or the like, or a display section for a liquid crystal television, a video integrated liquid crystal television or the like, and in various other fields. The liquid crystal display device basically comprises a backlight section and a liquid crystal display element section. As the backlight section, a type of an edge light system has frequently been used with a view to making the liquid crystal display device compact. Conventionally, as a backlight, a type has been in wide use in which at least one end face of a rectangular platelike light guide is used as a light incident end face, a linear or bar-shaped primary light source such as a straight tube fluorescent lamp is arranged along the light incident end face, a light emitted from the primary light source is entered into the light guide through the light incident end face thereof, and the light is emitted from a light emitting face which is one of two principal surfaces of the light guide.
Meanwhile, there has recently been made a demand for miniaturization and a reduction in power consumption regarding liquid crystal display devices of relatively small screen sizes such as portable electronic devices, e.g., cellular phone, personal digital assistants, portable game machines, etc., or indicators of various electric devices or electronic devices. Thus, an LED that is a point light source is used as a primary light source of the backlight in order to reduce power consumption. As the backlight that uses the LED as the primary light source, an example is described in JP(A)-7-270624 in which a plurality of LED's are one-dimensionally arrayed along a light incident end face of a light guide in order to exhibit a function similar to that of the backlight which uses the linear primary light source. The use of the primary light source based on the one-dimensional arrangement of the plurality of LED's enables acquisition of a required light amount and uniformity of a luminance distribution on an entire screen.
However, in the case of the compact liquid crystal display device, a further reduction is demanded in power consumption. In order to meet this demand, it is necessary to reduce the number of LED's to be used. However, a reduction in the number of LED's brings about lowering of luminance and non-uniformity of a luminance distribution.
JP(A)-9-133918 and JP(A)-10-20121 propose a planar light source device to achieve low power consumption without lowering luminance as much as possible. In the device, a rough surface or a lens surface is formed, on which a number of elongated lenses such as elongated prisms are formed in parallel with an incident end face as a light emitting mechanism on a light emitting face or a back surface of a light guide, and a prism sheet having a number of elongated prisms is arranged on the light emitting face of the light guide so as to set the prism surface of the prism sheet on the light guide side. According to the device, power consumption of the backlight is reduced and a distribution of an emitted light is narrowed to prevent lowering of luminance as much as possible.
In the case of applying such a light emitting structure to the planar light source device that uses the point light source, the device in which the rough surface is formed as the light emitting structure can emit a highly directive light of a narrower emitted light distribution in a plane (perpendicular direction) perpendicular to both of the light incident end face and the light emitting face of the light guide, but cannot sufficiently prevent a reduction of luminance because of a very wide emitted light distribution in a plane (parallel direction) parallel to the light incident end face of the light guide and perpendicular to the light emitting face. On the other hand, the device in which the lens surface is formed as the light emitting structure can emit a highly directive light of a narrower emitted light distribution in both of parallel and perpendicular directions to achieve high luminance, but has a problem of an extremely narrow distribution range of an emitted light, i.e., several to several tens of degrees, in a plane parallel to the light emitting face. Additionally, since the reduced number of LED's increases a distance between light emitting points, a region of the light guide close to a region between adjacent light emitting points is expanded to lower intensity of a light emitted therefrom in a required direction. This condition has a problem of bringing about non-uniformity (i.e., non-uniformity of uniformity degree of luminance) of a luminance distribution in an observation direction on a light emitting surface of the planar light source device.
An object of the present invention is to provide a high-quality planar light source device by solving, without deteriorating an emitted light distribution, the aforementioned problems of a reduction of luminance and non-uniformity of luminance distribution brought about by the use of a small number of primary light sources in a point state to achieve low power consumption of the planar light source device that uses the primary light source in the point state.